Oil recovery using EOR processes conventionally involves the injection of fluids under pressure into an oil bearing stratum in the vicinity of production wells. Desirably, the fluids drive oil from the pores and other interspecies within the stratum, thereby achieving an enhanced recovery of the desirable hydrocarbons. The fluids may be heated for improvement of the process.
A major difficulty encountered in enhanced oil recovery processes that involve the injection of fluids is the phenomenon of channeling. Under the high pressure gradients associated with injection, the injected fluids, which are usually less viscous than the oil in the pores, preferentially flow within the channels (“worm holes”) present within the formation with greater permeability relative to the target hydrocarbons, leading to early breakthrough of fluids to nearby production wells. A large number of patented processes have been developed in order to try to reduce this phenomenon of early breakthrough. These processes involve viscosity-graded fluids, special blocking agents, foams that set with time, and so on. However, all of these various processes have deficiencies in practice. The re-initiation of high-pressure injection usually leads once again to breakthrough, either in the same channel, or in other paths of higher relative permeability. It should be pointed out that this pressure-induced breakthrough is an entirely natural process characteristic of heterogeneous reservoirs. (Note that channeling in high-permeability streaks is not the same as viscous fingering, which occurs even in completely homogeneous reservoirs.)
It is an object of the present invention to achieve an improved EOR process, which at the same time disposes of various wastes within an oil bearing stratum, thereby achieving a generally permanent disposal of wastes. It is contemplated that the dual results thereby achieved will result in significant economic benefits, thereby to rendering oil recovery economically feasible in heretofore uneconomic deposits.
It has previously been proposed to enhance oil production by injecting sand or other wastes within the oil bearing stratum. However, this idea has been rejected, largely due to poor understanding of the nature of the injected sand bodies and other processes within the reservoir. However, when directed to appropriate target strata and with appropriate monitoring, EOR may be achieved by means of injecting sand or other wastes within a target stratum.
The development by the present inventors and others of the Slurry Fraction Injection (herein “SFI”) (TM) process to economically dispose of sand and waste liquids raises the possibility of deliberate fracture-inducing injection of wastes within appropriate reservoirs as a means of increasing the recoverable oil, or aiding other recovery processes. These include cold heavy oil production (“CHOP”), horizontal well extraction or thermal oil production stimulation. Of particular interest is the use of the large volumes of waste sand generated during CHOP for enhancement of oil extraction.
Successful SFI disposal of aqueous slurries containing sand and waste liquids from oil production operations such as stable emulsion and slops has been achieved, wherein substantial volumes of sand and waste liquids have been injected                into target formations. Such projects range in depth from 360 meters to 1260+meters, in formations that are either oil free, depleted, or that contain uneconomic amounts of oil in the pore space. A typical SFI project involves daily injection episodes of up to 800 cubic meters of slurry over a time interval of 8-12 hours. The slurry has a density of approximately 1200 kg/m3, and is injected under pressure that exceeds the overburden weight in order to achieve fracturing within the formations. It has proven feasible to add substantial amounts of liquid wastes containing viscous oil.        
Different target formations have been used for SFI at different sites. The most suitable formations are quartz-rich permeable and unconsolidated sandstones. These sands have permeabilities as high as 2-4 Darcy. In other target formations, comprised of finer grained sandstones, permeability values on the order of 0.5 to 2 Darcy have been measured. Individual wells used for SFI have, in some cases, received total sand volumes in excess of 30,000 cubic meters of sand, along with smaller volumes of oily waste fluids.
Careful monitoring of the SFI process both during and after injection episodes leads to the conclusion that the sand and viscous oil wastes are remaining relatively close to the injection well. Transmission of sand, oily wastes, or aqueous carrying fluids to higher formations seems not to occur, providing the SFI operations are conducted properly. Analysis of the injection pressures during active SFI shows that the solid material and the viscous fluids are entering the formation in the form of discrete fractures. Because the formations generally have a high permeability, the aqueous portion of the slurry is rapidly dissipated into the surrounding porous medium under the influence of the local high pressure gradients. These high pressure gradients are an actual consequence of the SFI process: the pressure in the slurry is somewhat higher than the overburden weight, and pressure in the liquid in the far field is much less than this value. For example, at 500 m depth, SFI pressures of 12-13 MPa are used, but the far-field pressures in the zones are about 4 MPa, giving very large pressure gradients that act to dissipate the pressures rapidly.
During disposal of wastes by means of SFI into oil-free sand strata, a region of lower permeability is generated around the wellbore. This low permeability region develops because the injected sand is usually more finegrained than the formation sand, and also contains approximately 2-5% viscous oil coating the sand grains. If waste fluids that contain large amounts of viscous oil are also injected at the same time, the permeability blocking effect is even greater. These conclusions are arrived at through careful analysis of the pressure decline curves following each SFI episode. In fact, the systematic analysis of pressure decline data forms a critical part of the monitoring and evaluation activity for SFI projects.